1. Field of the Invention
The present invention relates to an improvement of antenna system for use in a ground trunk system and so on.
2. Description of the Prior Art
Among the prior art antenna systems of the type which includes a main reflector, subreflectors and a primary radiator, there has been developed an improvement having wide-angle radiation characteristics by employing an anti-symmetrical mirror and disposing the subreflectors, primary radiator and a supporting pole in such a manner not to cause blocking.
This prior art antenna system, however, has a drawback in that a cross-polarized component can be generated because the mirror is not of rotation symmetry. To eliminate the above described defect, there has been developed another conventional antenna system which is provided with a conical horn 1 having a phase center F.sub.0 as a primary radiator, a first subreflector 2 having the phase center F.sub.0 of the conical horn 1 in common and further a focal point F.sub.1, a second subreflector 3 having the focal point F.sub.1 in common and further a focal point F.sub.2 and a main reflector 4 having a focal point F.sub.2 in common, as shown in FIGS. 2(a) and 2(b).
In the antenna systems shown in these figures, the first and second subreflectors 2 and 3 are a rotary elliptic reflector and rotary hyperbolic reflector, respectively. Further, the main reflector 4 is a rotary parabolic reflector. For the purpose of suppressing the generation of the cross-polarized component due to the anti-symmetry of the mirror, each of these prior art antenna systems are geometro-optically constructed such that the system of the reflectors is equivalent to a parabolic antenna as shown in FIG. 3. In this figure reference numeral 5 indicates a parabolic mirror having rotation symmetry.
However, in this antenna system, the cross-polarized component can be completely suppressed in case wherein the frequency of a beam emitted therefrom is infinite because the system of the mirrors is geometro-optically designed as described above. Thus, in a practical working frequency band such as microwave and millimeter-wave bands, the cross-polarized component generated due to the anti-symmetry of the reflectors cannot be completely eliminated. As a result, this conventional antenna system has a further drawback in that the cross-polarization characteristics thereof are deteriorated in the working frequency band.
Further, it is described in the Japanese Registered Patent Nos. 1361802 and 1364819 (refer to the Japanese Patent Application Publication Nos. 28247/1986 and 29570/1986 Official Gazettes) that geometro-optically obtained conditions of suppressing the cross-polarized component are given by: EQU e.sub.1.sup.2 -1=-{4L.sub.0 lsin.sup.2 [(.theta..sub.0 -.alpha.)/2]}(L.sub.0 +l).sup.2 (p 1)
and EQU e.sub.2.sup.2 -1={4sin[.beta.-.gamma.)/2]sin[(.alpha.-.gamma.)/2]sin(.beta./2)sin(.alpha ./2)}/sin.sup.2 [9.alpha.+.beta.-.gamma.)/2] (p2)
where e.sub.1 and e.sub.2 denote eccentricities of the curvatures of the first and second subreflectors, respectively, and furthermore .gamma. is given by EQU tan{(.gamma.-.alpha.)/2={L.sub.0 tan(.theta..sub.0 -.alpha.)/2}/(l+L.sub.0)(p3).
These conditions of suppressing the cross-polarized component in the prior art differ from those of suppressing the cross-polarized component according to the present invention which are obtained by taking the wave nature of the electric wave into consideration and will be described hereinafter. First, in the conditions of suppressing the cross-polarized component in the prior art, the eccentricities e.sub.1 and e.sub.2 of the subreflectors which satisfy the geometro-optically obtained conditions of suppressing the cross-polarized component are determined only by the geometrical arrangement of the subreflectors positions of which are represented by, for example, polar coordinates. Further, the arrangement of the reflectors are limited in such a manner that they have common focal points. Moreover, the geometro-optical technique of suppressing the cross-polarized component is carried out to eliminate the cross- polarized component in the frequency range of light and has a disadvantage in that, in the frequency range of microwave, such technique cannot suppress all of the cross-polarized components.
It is therefore a primary object of the present invention, which is accomplished to obviate such defects of the prior art, to provide an improved antenna system which has a preferable cross-polarization characteristics in the working frequency band.